1. Field of the Invention
The present invention relates to a surface acoustic wave device such as a surface acoustic wave resonator or a surface acoustic wave filter adapted to make use of surface acoustic waves and, more particularly, to a surface acoustic wave device having an improved electrode structure including bonding wires.
2. Description of the Related Art
Ordinarily, in a surface acoustic wave device such as a surface acoustic wave resonator or a surface acoustic wave filter adapted to make use of surface acoustic waves, a surface acoustic wave element is provided in a case. FIG. 10 shows an example of such a surface acoustic wave device. In a surface acoustic wave device 51, a surface acoustic wave element 53 is provided in a case 52. The case 52 has a stacked structure in which a case member 52b and a frame member 52c each having a rectangular frame shape and made of an insulating ceramic and a lid member 52d are stacked on a case member 52a which is formed of an insulating ceramic such as alumina. The case member 52a has a recess formed in its upper surface, and the surface acoustic wave element 53 is fixed on the bottom surface of the recess 52d via an adhesive material 54. The adhesive material 54 is an epoxy adhesive or the like and has a high strength in a set state in order to reliably fix the surface acoustic wave element 53.
External electrodes 55a and 55b extending to outer side surfaces are disposed on an upper surface of the case member 52a. The external electrodes 55a and 55b are connected to electrodes 53a and 53b on the surface acoustic wave element 53 via bonding wires 56a and 56b.
In the surface acoustic wave device 51, the adhesive material 54 used to fix the surface acoustic wave element on the case member 52a contracts during setting so as to have a reduced size and coverage area. Such shrinkage during curing causes a stress which acts on the bonded surfaces of the case member 52a and the surface acoustic wave element 53 in a tensile direction, as indicated by the arrows in FIG. 11, resulting in a variation in a characteristic of the surface acoustic wave device 51.
Also, a strain may be caused in the case 52 by application of an external force or the like to the case 52. In such an event, a strain acts on the surface acoustic wave element 53 through the adhesive material 54. That is, if, for example, a bending strain is caused in the case 52, a tensile stress or compressive stress acts on the surface of the surface acoustic wave element 53, as indicated by the arrows in FIG. 10. It is possible that such tensile stress or compressive stress will cause a variation in a characteristic of the surface acoustic wave device 51.
That is, the conventional surface acoustic wave device 51 has the problem of a variation in characteristic due to curing shrinkage or stress in the adhesive material 54 or strain in the case 52. Such a stress can be caused not only at the time of manufacturing but also during use of the surface acoustic wave device 51. There is, therefore, a possibility of occurrence of a change in characteristic of the surface acoustic wave device during actual use.
In addition, the bonding strength of the bonding wires 56a and 56b joined to the electrodes 53a and 53b of the surface acoustic wave element 53 via ultrasonic bonding is often insufficient to provide a reliable connection therebetween. Therefore, as shown in FIG. 12, the method of forming an electrode layer 53c of Cr and an electrode 53d of Au so as to be reliably joinable to the bonding wire on the electrode portion of the surface acoustic wave element 53 to which the bonding wire 56a is to be joined, and the method of increasing the film thickness of the electrode portion to which the bonding wire 56a is joined, as shown in FIG. 13, have been tried. Each of these methods, however, increases the manufacturing cost because the number of electrode forming steps is increased.